The present invention relates in general to telecommunication techniques. More particularly, the invention provides a system and method for generating optical return-to-zero signals with alternating bi-phase shift and frequency chirp. Merely by way of example, the invention is described as it applies to optical networks, but it should be recognized that the invention has a broader range of applicability.
Telecommunication techniques have progressed through the years. As merely an example, optical networks have been used for conventional telecommunications in voice and other applications. The optical networks can transmit multiple signals of different capacities. For example, the optical networks terminate signals, multiplex signals from a lower speed to a higher speed, switch signals, and transport signals in the networks according to certain definitions.
In optical communications, an optical signal may transmit a long distance, such as hundreds or even thousands of kilometers, in optical fiber links. The quality of received signals often can be improved by using return-to-zero (RZ) modulations instead of non-return-to-zero (NRZ) modulations. For example, a signal under return-to-zero modulation includes logic low and high states, such as ones represented by “0” and “1” respectively. The signal state often is determined by the voltage during one part of a bit period, and the signal returns to a resting state during another part of the bit period. As an example, the resting state is represented by zero volts. In another example, a signal under non-return-to-zero modulation includes logic low and high states, such as ones represented by “0” and “1” respectively. The signal state often is determined by the voltage during a bit period without the signal returning to a resting state during at least a part of the bit period.
The return-to-zero modulations usually can provide better resistance to signal noises than the non-return-to-zero modulations. Additionally, the isolated RZ pulses often experience nearly identical nonlinear distortions during transmission, which can be at least partially mitigated through proper dispersion compensation schemes. Hence RZ signals usually are more resistant to nonlinear distortions than NRZ signals.
FIG. 1 is a simplified conventional system for generating NRZ signals. The system 100 includes an NRZ source 110, an NRZ data driver 120, a continuous wave (CW) diode laser 130, and a data modulator 140. In contrast, the conventional system for generating RZ signals is often more complicated as shown in FIGS. 2, 3, and 4.
FIG. 2 is a simplified conventional system for generating RZ signals. The system 200 includes an NRZ source 210, a converter 215, an RZ data driver 220, a CW diode laser 230, and a data modulator 240. The data modulator 240 is an electro-optical (EO) modulator. The converter 215 can convert an NRZ signal to an RZ signal in electrical domain. The electrical RZ signal is then used to generate an optical RZ signal through the EO modulator 240. The EO modulator 240 can be either a Mach-Zehnder (MZ) modulator or an electro-optical absorptive modulator. The system 200 often generates simple RZ signals that contain no phase or frequency modulations.
FIG. 3 is another simplified conventional system for generating RZ signals. The system 300 includes an NRZ source 310, an NRZ data driver 320, a CW diode laser 330, a data modulator 340, a clock driver 350, a phase shifter 355, and a clock modulator 360. The data modulator 340 and the clock modulator 360 each are an EO modulator. The EO modulator 360 is driven by a data clock signal or a half-rate data clock signal, and is used to generate optical clock pulses. FIG. 4 is yet another simplified conventional system for generating RZ signals. The system 400 includes an NRZ source 410, an NRZ data driver 420, a directly modulated laser 430, a data modulator 440, a clock driver 450, and a phase shifter 455. The laser 430 is directly modulated with a data clock signal to generate optical clock pulses. With proper arrangements, phase or frequency modulations can be added to the optical clock pulses to generate complex RZ signals.
Among complex RZ signals, the optical carrier-suppressed return-to-zero (CSRZ) signals can provide strong transmission capabilities. For example, the CSRZ signals have alternating bi-phase shifts between adjacent bits, and are less affected by inter-symbol interferences than the simple RZ signals. Thus the CSRZ signals are more tolerant for both dispersions and nonlinear distortions. In another example, the chirped return-to-zero (CRZ) signals have substantially the same frequency chirp on each RZ pulse for a given signal. The frequency chirp can be made to compensate for the chirp induced by nonlinear effects, and further improve tolerance for nonlinear distortions. But the conventional systems for generating these RZ signals often are complex and expensive.
Hence it is highly desirable to improve techniques for generating return-to-zero signals with frequency chirp.